The emulsion or suspension polymerization of ethylenically unsaturated monomers can be carried out by batch, semibatch or continuous processes. Batch or semibatch processes are usually carried out in stirred reactors and continuous processes are carried out in cascades of stirred vessels, tube reactors or loop reactors. Thus, U.S. Pat. No. 3,551,396 describes processes for preparing polyvinyl acetate dispersions in a loop reactor, in which monomer and water are continuously circulated and additional monomer and initiator are continually metered in and polymer formed is continually taken off from the reactor. U.S. Pat. No. 2,703,794 describes processes for the continuous emulsion polymerization of vinyl acetate and ethylene in the presence of emulsifier and optionally a water-miscible organic solvent. Disadvantages are oscillating processes parameters and consequently formation of polymers having different particle size distributions. DE-A 2555142 describes a continuous process for preparing polymers of ethylenically unsaturated monomers by the emulsion polymerization process in a plurality of polymerization reactors connected in series. EP-A 834518 describes a batch polymerization process in which the polymerization proceeds in a reactor having an external cooling circuit, in which it is absolutely necessary to use a low-shear pump in order not to adversely affect the products. WO-A 03/006510 describes a batch polymerization process in a polymerization reactor having an external cooling circuit, in which the polymerization batch is circulated by pumping by means of a low-shear cylinder pump or peristaltic pump in order for the products not to be adversely affected by shearing. WO-A 02/59158 describes a polymerization process in a reactor having an external cooling circuit, in which the polymerization batch is circulated by pumping and at least a partial amount of the monomer has to be introduced into the external circuit. A disadvantage of the batch polymerization processes in which large-volume batches containing dispersed polymer particles have to be circulated by pumping is their susceptibility to coagulation as a result of the shear forces introduced by the pump.
However, to prepare polymers having different average particle sizes or different particle size distributions, it is generally necessary to vary the formulations of the polymerization components or make substantial changes in the polymerization conditions or polymerization process.
Furthermore, it is a problem, especially in the case of continuous emulsion or suspension polymerization processes, to set constant polymerization conditions over the entire duration of the polymerization and thus prevent oscillation of the process parameters so that polymers having the same properties are formed over the entire duration of the polymerization.